The present invention is related to wireless data communication networks and, in particular, relates to controlling access to network resources.
Conventional wireless networks utilize various network access protocols in order to allow communicating nodes (i.e. radios) to share network resources. Wireless VHF communication networks typically include multiple nodes communicating over a single radio channel.
Conventional protocols generally permit each node to transmit over the radio channel without restriction. In one such protocol, the destination node sends an acknowledgment message upon successfully receiving the transmitted message. The source node waits for the acknowledgment message, and if the acknowledgment message is not received after a predetermined time period, the source node assumes that the transmitted message was not received and retransmits the message. Because nodes may transmit without restriction on the same channel, conflicts or collisions may occur that prevent the transmitted message from being received. A collision occurs whenever two or more nodes attempt to transmit simultaneously.
A typical communication system with five nodes communicating over a single channel according to this protocol is disclosed in FIG. 1. The timeline (t) of channel transmissions shows that during time period 10 a message is transmitted from node 1 to node 2. During the time period 20 a message is transmitted from node 3 to node 5. During the time period 30 a message is transmitted from node 4 to node 5. No protocol is in place to prevent collisions. The nodes transmit whenever there is a message to be sent. As a result, collisions between transmissions will occur. For example, node 4 is shown as attempting to transmit a message to node 1 during time period 15 which will collide with the transmission from node 1 to node 2 during time period 10. Similarly, the transmission from node 4 to node 2 during time period 25 will collide with the transmission from node 1 to node 2 during time period 10. These collisions may prevent either message from being received, resulting in the inefficient requirement for retransmissions.
An existing network access protocol attempts to avoid collision. The Carrier Sense Multiple Access (CSMA) protocol, requires that a source node having a message to broadcast over the common channel first “listen” for activity on the channel. If activity is detected (i.e. another node is transmitting), then the source node waits for a time period before re-attempting transmission. The CSMA protocol reduces the need for retransmissions, since collisions are rare. However, retransmissions may still be required if the receiving node is not available because, for example, it is out of range or powered off.
Another network access protocol is Time Division Multiple Access (TDMA). The TDMA protocol eliminates collisions by dividing the common channel into time frames and slots. Each node is assigned a particular time slot in which to transmit all of its messages. Collisions are eliminated because nodes only transmit during their assigned slots. However, the TDMA protocol may result in excessive delays and under utilization of the common channel, since each node must wait for its designated time frame and time slot to transmit.
Other wireless communication systems use multiple channels. For example, in a cellular digital packet data system multiple channels are provided permitting more than one pair of nodes to exchange data simultaneously. In addition to the communicating nodes, the system also includes base stations that control communications between the nodes. Each base station covers a particular geographic service area and controls communications to and from nodes located in that area. In order to deliver a message the communication system must first locate the destination node. To locate a destination node, the network broadcasts a location message from each of the base stations in the service area until the destination node responds. This procedure dedicates a large amount of network resources to locating nodes. In addition, failure or elimination of the base station generally causes a loss of all communications.
Conventional multi-channel wireless communication systems typically include a control channel and at least one data channel. The data channel is used for voice and data transmissions between pairs of nodes. The control channel is used for the transmission of control data between a base station and a node. Such control data may include, for example, instructions for the node to change to a particular data channel in order to receive an incoming transmission from another node. In conventional systems, nodes do not communicate with each other over the control channel.
As discussed above, the current methods of providing channel access in wireless networks have many shortcomings. It is an object of the present invention to obviate many of these shortcomings and to provide a novel wireless communication system and method for providing efficient channel access.
Accordingly, it is an object of the present invention to provide a novel system and method for communicating between nodes of a single channel wireless network using a channel access protocol that avoids the need for a large number of message retransmissions.
It is another object of the present invention to provide a novel system and method for communicating between nodes of multiple channel wireless network using a channel access protocol that avoids the need for a large number of message retransmissions.
It is yet another object of the present invention to provide a novel system and method for communicating between nodes of a single channel wireless network using a channel access protocol that reduces collisions between messages.
It is still another object of the present invention to provide a novel system and method for communicating between nodes of multiple channel wireless network using a channel access protocol that reduces collisions between messages.
It is a further object of the present invention to provide a novel system and method for communicating between nodes of a single channel wireless network using a hybrid CSMA/TDMA channel access protocol.
It is yet a further object of the present invention to provide a novel system and method for communicating between nodes of multiple channel wireless network using a hybrid CSMA/TDMA channel access protocol.
It is still a further object of the present invention to provide a novel system and method for VHF communications that provides for communications between multiple nodes without the use of a base station.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.